Optical memory technology using an optical recording medium having a pit-like pattern as a high-density and large-capacity memory medium has been practically used while having extended applications such as a digital versatile disk (DVD), a video disk, a document file disk, and further a data file.
In recent years, in order to further highly densify recording density of an optical recording medium, it has been investigated to enlarge numerical aperture (NA) of an objective lens which converges a light beam on an optical recording medium to form a diffraction-limited minute spot. However, spherical aberration resulting from an error of thickness of a protective layer protecting a recording layer of an optical recording medium is proportional to the biquadrate of NA. As for an error of thickness, in particular in a high-density disk like a Blu-ray disk, since thickness of an original protective layer is thin (it is 0.1 mm in the case of a Blu-ray disk), it becomes impossible to disregard an effect which appears as a spherical aberration even if an absolute value of dispersion is very small. Hence, in the case of making NA large at such as 0.8 or 0.85, it becomes indispensable to provide instrument which connects the spherical aberration in the above-mentioned optical system.
There is multilayering of an optical recording medium as a method of further enlarging storage capacity of an optical recording medium. Generally, in an optical recording medium which has a plurality of recording layers, since an interlayer is arranged between respective recording layers, amounts of spherical aberrations generated at the time of focusing respective recording layers differ. For this reason, it becomes indispensable to correct a spherical aberration at every recording layer which is focused.
Then, in particular, as structure for correcting a spherical aberration generated by thickness dispersion of a protective layer of an optical recording medium, and a manufacturing error of a lens, Japanese Patent Laid-Open No. 2000-131603 proposes inserting an expander lens, which is constituted of two lenses, between a laser and an objective lens to perform variable adjustment of a space between the two lenses which constitute this expander lens.
An example of a conventional optical information apparatus mentioned above will be explained with reference to drawings here.
FIG. 12 is a schematic diagram showing structure of the conventional optical information apparatus, which is constituted of an optical head 1, focus control instrument 19, tracking control instrument 20, laser power control instrument 21, reproduced signal processing instrument 102, and a controller 103.
In addition, the optical head 1 is constituted of a laser 2, a diffraction grating 3, a collimator lens 4, a polarization beam splitter 5, a mirror 7, a quarter wavelength plate 8, an objective lens 9, a condenser lens 11, a cylindrical lens 12, a photodetector 13, an opening 16 for an objective lens, and an actuator 17. Furthermore, spherical aberration correction instrument 24 is constituted of a lens group 6 and driving instrument 18.
Here, the laser 2 is a laser which is constituted of, for example, a GaN-based semiconductor laser element (wavelength of 405 nm), and outputs coherent light for record and reproduction to a recording layer of an optical recording medium 10. The diffraction grating 3 is an optical element in which a concavo-convex pattern is formed on a surface of a glass substrate, and which divides an incident beam into three beams to enable detection of a tracking error signal by a so-called three-beam method.
The collimator lens 4 is a lens which converts divergent light, emitted from the laser 2, into parallel light. The polarization beam splitter 5 is an optical element whose transmittance and reflectivity change in a polarized direction of incident light, and which separates the light. The spherical aberration correction instrument 24 is an instrument which connects a spherical aberration generated by thickness dispersion of a protective layer and the like of the optical recording medium 10, and which is constituted of a concave lens 6a, a convex lens 6b, and driving instrument 18, and it is possible to correct the above-mentioned spherical aberration by changing a space between the concave lens 6a and convex lens 6b. The mirror 7 is an optical element which reflects incident light to make it go in a direction of the optical recording medium 10, and has characteristics of 5% of transmission and 95% of reflection of some linearly polarized light, and 100% of reflection of linearly polarized light which is orthogonal to the above-mentioned linearly polarized light.
The quarter wavelength plate 8 is formed with a birefringence material, and is an optical element which transforms linearly polarized light into circularly polarized light. The objective lens 9 is a lens which condenses light to a recording layer of the optical recording medium 10, and whose numerical aperture (NA) is 0.85. The condenser lens 11 is a lens which condenses light, reflected by the recording layer of the optical recording medium 10, to the photodetector 13.
The cylindrical lens 12 whose incident plane is a cylindrical face and whose outgoing plane is a rotationally symmetric face to a lens optical axis, provides the astigmatism of enabling detection of a focus error signal by a so-called astigmatism method to incident light.
The photodetector 13 receives light reflected by the recording layer of the optical recording medium 10, and converts the light into an electric signal.
The opening 16 for an objective lens is for restricting the size of light incident into the objective lens 9, and determining NA of the objective lens, and also serves as a member holding the objective lens 9. The actuator 17 performs focus control which is position control in a direction of an optical axis, and tracking control which is position control in a direction vertical to it, and is constituted of driving instrument such as a coil and a magnet. The driving instrument 18 drives the concave lens 6a in the direction of the optical axis.
The operation of the optical information apparatus constituted in this way will be explained. The linearly polarized light emitted from the laser 2 is divided into three beams by the diffraction grating 3, and this light divided into three beams is transformed into parallel light by the collimator lens 4. The light made into parallel light permeates the polarization beam splitter 5, and is incident into the lens group 6. Here, in order to correct a spherical aberration generated by dispersion of protective layer thickness of the optical recording medium 10, the incident parallel light is transformed into divergent light or convergent light by changing the space between the concave lens 6a and convex lens 6b, which constitute the spherical aberration correction instrument 24, using the driving instrument 18, and this transformed light is incident into the mirror 7, its part penetrates, and most is reflected to be changed for its traveling direction toward the optical recording medium 10. This reflected light is incident into the quarter wavelength plate 8, the linearly polarized light is transformed into circularly polarized light, this circularly polarized light is limited for an opening by the opening 16 for an objective lens to be incident into the objective lens 9, generates a spherical aberration according to a divergent degree or a convergent degree of the incident light, and is condensed on the optical recording medium 10. Here, in order to correct the spherical aberration generated on the recording layer by the dispersion in the thickness of the protective layer of the optical recording medium 10, light which has a spherical aberration in a direction which cancels the spherical aberration resulting from the thickness of the protective layer is condensed by the objective lens 9, and hence, a light spot which has no aberration, that is, which is stopped down to a diffraction limit is formed on the recording layer of the optical recording medium 10.
Next, the circularly polarized light reflected from the optical recording medium 10 is incident into the quarter wavelength plate 8 to be transformed into linearly polarized light in a direction orthogonal to the linearly polarized light which is emitted from the laser 2. The linearly polarized light transformed by the quarter wavelength plate 8 is altogether reflected by the mirror 7, permeates the lens group 6, is reflected by the polarization beam splitter 5, is converged by the condenser lens 11 without returning to the laser 2, is given astigmatism by the cylindrical lens 12, and is condensed on the photodetector 13. The photodetector 13 transforms the received light beam into an electric signal. This electric signal is supplied to the focus control instrument 19, tracking control instrument 20, and reproduced signal processing instrument 102.
The focus control instrument 19 obtains a focus error signal from the signal supplied from the photodetector 13, and performs focus control, which is position control in the direction of the optical axis, using the actuator 17 according to this focus error signal. The tracking control instrument 20 obtains a tracking error signal from the signal supplied from the photodetector 13, and performs tracking control using the actuator 17 according to this tracking error signal so that the light beam may get on-track in a predetermined area on the optical recording medium 10. In addition, the focus error signal and tracking error signal are detected by widely known technology, for example, an astigmatism method and a three-beam method.
A reproduced signal according to recording information recorded on the optical recording medium 10 is supplied to the reproduced signal processing instrument 102. The reproduced signal processing instrument 102 performs processing such as waveform equalization to this reproduced signal, and outputs the reproduced data as digital data.
Here, it will be described in detail that spherical aberration correction becomes possible using the spherical aberration correction instrument 24. When the space between the concave lens 6a and convex lens 6b which constitute the spherical aberration correction instrument 24 is narrowed, parallel light is transformed into divergent light, and when the space is enlarged, it is transformed into convergent light. That is, it is possible to generate light, which has a divergent/convergent angle with a different positive/negative sign on the basis of parallel light, by freely changing the divergent angle of light outputted from the spherical aberration correction instrument 24 by changing the space between the concave lens 6a and convex lens group 6b. Here, when divergent light or convergent light, i.e., non parallel light which has an elevation angle or an depression angle to the optical axis is incident into the objective lens 9, a spherical aberration arises in the light stopped down by the objective lens 9, and its size and direction depend on an angle of incident divergent light/convergence light (an elevation angle/depression angle), and hence, it becomes possible to correct the spherical aberration, generated by the base material thickness dispersion of the optical recording medium 10 or the like, by using this spherical aberration.
On the other hand, in an optical information apparatus, in order to perform suitable reproduction or record, it is necessary to perform control such as optimization of laser power irradiated on the optical recording medium 10. Then, luminous intensity taken out in an arbitrary location of an optical system from the laser to the objective lens is measured, and feedback control which controls largeness of an output of laser power is performed on the basis of the luminous intensity.
Nevertheless, when it is attempted to perform feedback control of laser power in the optical information apparatus with such structure as shown in the above-mentioned FIG. 12, the following malfunctions are supposed.
FIG. 13 is a diagram showing structure of performing feedback control of laser power in the optical information apparatus equipped with the above-mentioned spherical aberration correction instrument 24.
In FIG. 13, a lens 14 condenses light, permeating the mirror 7, to a light quantity detector 15. The light quantity detector 15 transforms the received light beam into an electric signal. In addition, an opening 14a for a lens adjusts the light incident into the lens 14. Furthermore, in the optical system, the opening 14a for a lens, lens 14, and light quantity detector are made to use one side of light at the time of being branched by the mirror 7 after passing the spherical aberration correction instrument 24.
In such structure, the light which permeates the mirror 7 is condensed to the light quantity detector 15 by the lens 14 through the opening 14a for a lens, and the light quantity detector 15 transforms the received light beam into an electric signal. This electric signal is a signal (outgoing power detection signal a) of monitoring the outgoing power of the laser 2, and is inputted into the laser power control instrument 21. On the other hand, the controller 103 sets the outgoing power of the laser optimum for reproduction or record, and it is inputted into the laser power control instrument 21 as a reference voltage signal b. The laser power control instrument 21 controls an amount of a laser driving current supplied to the laser 2 so that the outgoing power detection signal a and reference voltage signal b may become equal. Thereby, the outgoing power of the laser 2 is controlled at predetermined power in any case of reproduction and record.
Nevertheless, in the above-mentioned structure, the following malfunctions are supposed. Hereafter, explanation will be performed using FIG. 14. FIG. 14 is a diagram showing schematically light which is incident into the objective lens 9 when the driving instrument 18 drives the concave lens 6a to correct a spherical aberration. In FIG. 14, when a protective layer of the optical recording medium 10 is thick, the space between the concave lens 6a and convex lens 6b of the spherical aberration correction instrument 24 becomes large so as to cancel the spherical aberration accompanying this thickness, and hence, the light which is reflected by the mirror 7 is incident into the objective lens 9 in convergent light. Continuous lines show this state.
In addition, when the protective layer thickness of the optical recording medium 10 is thin, on the contrary to the above-mentioned case, the space between the concave lens 6a and convex lens 6b becomes narrow, and the light reflected by the mirror 7 is incident into the objective lens 9 in divergent light. The dotted lines show this state.
In the above structure, when the concave lens 6a moves so as to correct the spherical aberration of the optical recording medium 10, it is necessary to make the quantity of light incident into the objective lens 9 constant in its moving range, that is, regardless of a location of the concave lens 6a. That is, it is necessary to prevent leakage light, which is not irradiated on the objective lens 9 as shown in dotted and dashed lines in FIG. 14, from arising. For that purpose, it is necessary to design beforehand optical arrangement of the concave lens 6a and convex lens 6b in the spherical aberration correction instrument 24, the quarter wavelength plate 8, objective lens 9, and the like.
Nevertheless, in the structure shown in FIG. 13, the above consideration has not been given about the structure in a side of the light quantity detector 15. In this case, the following situations arise.
Namely, supposing that the opening 14a for a lens, that is, light used for the light quantity detector 15 is in a position A shown in FIG. 14, light quantity incident into the light quantity detector 15 (area of a substantial light-receptive region) varies according to a position of the concave lens 6a since shading and the like arise as shown by optical paths by continuous lines and optical paths by dotted lines which are in the figure. That is, although the outgoing power of the laser 2 does not change, since a cross-sectional area of a light beam to a side of the light quantity detector 15 changes by the correction of the spherical aberration correction instrument 24, the incident light quantity to the light quantity detector 15 changes.
For this reason, for example, as shown in FIG. 15, even if the outgoing power of an objective lens output is made constant, an output level (outgoing power detection signal a) of the light quantity detector 15 changes to Vdet1 to Vdet2 (Vdet2>Vdet1) according to the spherical aberration correction amount SA1 to SA2. Here, since the laser power control instrument 21 performs control so that this outgoing power detection signal a becomes equal to the reference voltage signal b, when laser power control is performed in this state, the outgoing power of the objective lens output changes according to the spherical aberration correction amount.
Thus, since a signal level detected by the light quantity detector 15 becomes small when the spherical aberration correction amount is SA1, it is controlled so that the outgoing power of the laser 2 may become large. At this time, while reproducing the optical recording medium 10, power more than needed is irradiated on the optical recording medium 10, and there is a possibility of accidentally degrading recorded information recorded on the optical recording medium 10. In addition, on the contrary, since the signal level detected by the light quantity detector 15 becomes large when the spherical aberration correction amount is SA2, it is controlled so that the outgoing power of the laser 2 may become small. At this time, while reproducing the optical recording medium 10, reproductive signal quality deteriorates, and there is a possibility that it may become impossible to reproduce recording information.
As opposed to such a malfunction, what is conceivable is such a measure of designing an optical system so as to be able to secure fixed light quantity also in a side of the light quantity detector 15 similarly to the side of the objective lens 9, or providing the optical detector 15 in a position where it can receive directly light which is not affected by the spherical aberration correction instrument 24, for example, light having passed the polarization beam splitter 5.
Nevertheless, the former measure imposes severe optical conditions in a design of the optical head 1 to both of designs of sides of the objective lens 9 and optical detector 15, and hence, itself is reflected in manufacturing cost. Furthermore, optimal size of satisfying both optical conditions is enlarged and when it cannot be obtained within, for example, the designed size of a conventional light head, a malfunction that a new parts design must be performed is caused. The same problem arises also in the latter.
In addition, the problem mentioned above arises similarly, when correcting a spherical aberration by moving a collimator lens in the direction of the optical axis.
Then, the present invention is proposed in view of the above-described actual conditions, and provides an optical information apparatus which not only can respond to dispersion in thickness of a protective layer of an optical recording medium without depending on optical conditions in a design, but also can keep outgoing power of an objective lens output constant, and a laser power setting method comprising such an optical information apparatus.